1. Field of the Invention
This invention relates generally to integrated circuits and, more particularly, to a photoresist technique for use in fabricating integrated circuits.
2. Description of the Related Art
Integrated circuits, such as microprocessors and memory devices, are used in a wide variety of applications. Such applications include personal computers, industrial control systems, telephone networks, and a host of consumer products, just to name a few. As most people are aware, an integrated circuit is a highly miniaturized electronic circuit that has revolutionized the functionality, dependability, and size of these various products.
In the manufacturing of integrated circuits, numerous microelectronic circuits are simultaneously fabricated on a semiconductor substrate. Such substrates are typically referred to as wafers, and a typical wafer includes a number of different regions, known as die regions. When the fabrication of the integrated circuits on the wafer is complete, the wafer is cut along these die regions to form individual die. Each die contains at least one microelectronic circuit, which is usually replicated on each die.
Although often referred to as semiconductor devices, integrated circuits are in fact fabricated from numerous materials of varying electrical properties. For example, integrated circuits are typically built upon a base substrate which is commonly made of semiconductive or insulative material. The characteristics of the base substrate may be altered, by etching or doping for instance, and various materials, such as insulators, dielectrics, conductors, and semiconductors, may be deposited on the base substrate in various patterns to form the final integrated circuit.
Of particular interest in this disclosure is the formation of openings in a portion of the substrate of the integrated circuit. Such openings are typically formed by fabricating a structure in a layer of insulative or dielectric material. Such structures may include, for instance, trenches, contact openings, vias, or containers. These various structures are used in most integrated circuits to form contacts between various circuit elements or to create various circuit elements.
Such structures are typically formed using a photolithographic process. Photolithography is a transfer process where the pattern on a photomask is replicated in a radiation-sensitive layer on the surface of the substrate. In a photolithographic process, a layer of radiation-sensitive material is deposited over the substrate, such as the base substrate or one or more other layers that have been deposited on the base substrate. For instance, when using ultraviolet (UV) light as the radiation source, photoresist, which is a UV-sensitive polymer, is used as the radiation-sensitive layer.
After the photoresist has been deposited onto the substrate, the substrate is placed in an exposure system, and the photomask pattern to be transferred is aligned with any existing patterns on the substrate. The photoresist is then exposed to UV radiation through the photomask. The radiation changes the structure of the photoresist in a manner that depends upon whether the photoresist type is positive or negative. Negative photoresist becomes polymerized, i.e., cross-linked, in areas exposed to the radiation, whereas, in a positive photoresist, the polymer bonds are broken upon exposure. In either case, the photoresist is not affected in regions where the photomask is opaque. After exposure, the photoresist is developed to dissolve the unpolymerized regions, while the polymerized portions of the photoresist remain intact to form a photoresist pattern on the substrate which is essentially identical to the pattern on the photomask.
After the photoresist pattern is formed on the surface of the substrate, portions of the substrate underlying the openings in the photoresist layer may be etched to form holes in the substrate, or dopants may be diffused or implanted into the exposed portions of the substrate. In regard to the first possibility, e.g., etching, it may be desirable to remove a portion of the substrate along a given profile. For example, it may be desirable to remove a portion of the substrate in a direction directly perpendicular to the surface of the substrate. In such a situation, an anisotropic etch may be performed. Alternatively, it may be desirable to remove a portion of the substrate that underlies the photoresist along the edges of the photoresist along with the exposed region. In this circumstance, an isotropic etch may be used, as it tends to undercut the photoresist.
However, it may also be desirable to create a stepped profile, e.g., a tapered or wine glass-shaped profile, in the underlying substrate. In this instance, the deeper portions of the hole in the substrate are narrower than the upper portions of the hole in the substrate. To create this type of stepped hole, a first photoresist pattern having a small hole is created over the substrate, and a first etching step is performed. Thereafter, a second photoresist pattern is created either by redeveloping the first photoresist layer using a different photomask or by removing the first photoresist layer and forming a second photoresist layer having larger openings. Once the revised photoresist pattern has been created, a second etching step is performed. Alternatively, the wider opening my be etched first such that it stops on an etch stop layer, and then the smaller opening may be etched.
This method is undesirable because it requires additional development steps, possibly including two complete photolithographic steps. Also, two photomasks are required, thus causing possible alignment problems.
The present invention may address one or more of the problems set forth above.